The invention relates to a non-linear voltage-dependent resistor having a ceramic sintered body based on zinc oxide as a resistance material which is doped with at least one alkaline earth metal, at least one rare earth metal and at least one metal of the iron group present as oxides and with at least one of the metals of the group aluminum, gallium and/or indium and electrodes provided on the oppositely located major surfaces of the sintered body. The invention also relates to a method of manufacturing such a resistor.
Non-linear voltage-dependent resistors (hereinafter also referred to as varistors) are resistors the electric resistance of which at constant temperature above a threshold voltage U.sub.A decreases very considerably with increasing voltage. This behaviour may be described approximately by the following formula: EQU I=(V/C).sup..alpha.
wherein:
I=current through the varistor PA1 V=voltage drop at the varistor PA1 C=geometry-dependent constant; it indicates the ratio voltage/(current).sup.1/.alpha..
In practical cases this ratio may take a value between 15 and a few thousands.
.alpha.=current index, non-linearity factor or control factor; it depends on the material and is a measure of the slope of the current-voltage characteristic; typical values are in the range from 30 to 80.
Varistors are frequently used for the protection of electrical devices, apparatuses and expensive components from excess voltage and voltage peaks. The operating voltages of varistors are in the order of magnitude from 3 V to 3000 V. For the protection of sensitive electronic components, for example integrated circuits, diodes or transistors, low-voltage varistors are increasingly required, the operating voltages U.sub.A of which lie below approximately 30 V and which show as high values as possible for the coefficient of non-linearity .alpha.. The higher the value for the coefficient of non-linearity .alpha., the better is the operation as an excess voltage limiter and the smaller is the power consumption of the varistor. Varistors based on zinc oxide show comparatively good efficients of non-linearity .alpha. in the range from 20 to 60.
Varistors based on zinc oxide and having approximately 3 to 10 mol. % metal oxide additions, for example, MgO, CaO, La.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Co.sub.3 O.sub.4 as a dopant are known (for example, from DE 29 52 884, or Jap. J. Appl. Phys. 16 (1977), pp. 1361 to 1368). As a result of the doping the interior of the polycrystalline ZnO grains becomes low-ohmic and high-ohmic barriers are formed at the grain boundaries. The contact resistance between two grains is comparatively high at voltages &lt;3.2 V but at voltages &gt;3.2 V it decreases by several orders of magnitude when the voltage increases.
Varistors with sintered bodies based on zinc oxide doped with rare earth metal, cobalt, boron, an alkaline earth metal and with at least one of the metals of the group consisting of aluminum, gallium and/or indium are known from DE 33 23 579.
Varistors with sintered bodies based on zinc oxide doped with a rare earth metal, cobalt, an alkaline earth metal, alkali metal, chromium, boron and with at least one of the metals of the group consisting of aluminum, gallium and/or indium are known from DE 33 24 732.
Both the varistors known from DE 33 23 579 and the varistors known from DE 33 24 732 only show useful values for the non-linearity coefficient .alpha. at threshold voltages U.sub.A above 100 V with .alpha.&gt;30. At threshold voltages U.sub.A below 100 V the values for .alpha. with the range from 7 to 22 are too low as regards effective excess voltage limit and power input of the varistors. Moreover, a boron doping has a flux activity and leads to the formation of liquid phases in the sintered body during the sintering process, which is undesired when diffusion processes must be avoided during the sintering.
The way usually employed so far of manufacturing low-voltage varistors based on doped zinc oxide is to use coarse granular resistance material. Sintered bodies of doped zinc oxide having a comparatively coarse granular structure with grain sizes &gt;100 .mu.m are obtained, for example, when material of the system ZnO--Bi.sub.2 O.sub.3 is doped with approximately 0.3 to approximately 1 mol. % of TiO.sub.2. TiO.sub.2 forms with Bi.sub.2 O.sub.3 a low-melting-point eutectic when sintering which stimulates the grain growth of polycrystalline ZnO. A disadvantage, however, is that comparatively long rod-shaped ZnO crystallites are often formed which considerably impede a control of the microstructure of the ceramic structure. The grain distributions which are always very wide and nearly always inhomogeneous in a TiO.sub.2 -doped resistance material from the system ZnO--Bi.sub.2 O.sub.3 nearly render the manufacture of varistors with reproducible operating voltage U.sub.A &lt;30 V substantially impossible.